Add crossref.fetch_references(doi), which parses message.reference[].DOI into Citations (DOI-less book/unstructured refs are skipped — they cannot join the graph). Wire it as the third leg of the ingest citation fallback chain: OpenAlex-empty -> S2 -> Crossref, for DOI papers only (Crossref's works endpoint is DOI-keyed). Cited DOIs are normalized to the canonical bare lower-case form via _norm_cited_id.
As a fallback it fires only when OpenAlex and S2 both return no references, so it adds a forward-looking third source without changing the already-covered corpus.
Tests: fetch_references unit tests (DOI edges / unstructured skipped / no refs / 404) and ingest tests for both fallback directions. Full suite 361 passed; ruff + mypy clean. Live-validated: 33/40/24 refs for Springer/ACM/Wiley DOIs.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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